Such belly markings are typically used for branding in air show flyby.
A flying debut is the signature moment in the development of any new plane, when the industry goes into publicity overdrive. A 600km trip from Toulouse to Le Bourget is surely the unannounced-target of Airbus; date of first flight is not certain, and those of the following flights are even more uncertain as you will never know what issues have to be resolved. So, if one of the test flights can be combined with a flyby at Le Bourget it would be a bonus, but not a priority.
A350 logo on the belly fairing